The present invention relates to a reduced beak planox process for the formation of integrated electronic components.
Planox technology, i.e. with oxide grown in selected zones of a plane surface of silicon, is commonly used for field insulation in MOS and CMOS processes because it provides self-alignment of insulation implantation and avoids steep steps at the edge.
It calls for the following sequence of process steps:
a. growth of thin oxide (30-90 nm) on a silicon substrate; PA0 b. deposit of a thin layer of silicon nitride (90-300 nm); PA0 c. deposit of a photoresist and masking; PA0 d. etching of the silicon nitride, typically in a plasma, selective toward the underlying oxide, with removal of nitride from the zone where the field oxide is to be grown; PA0 e. removal of the photoresist; PA0 f. optional removal of the initial oxide from the zone no longer covered with nitride and growth of the field oxide; PA0 g. removal of the residual nitride and the underlying initial oxide and definition of the active areas not provided with field oxide.
The major drawback of present planox technology lies in the so-called `bird beak`, i.e. the growth of oxide wedges under the nitride at the edges of the area it covers. This phenomenon involves a loss of 0.5-1 micron of the nominal dimensions of the active area and hence limits the density of the circuits.
It is known from the literature that the length of the planox beak is directly proportionate to the thinness of the nitride and the thickness of the underyling oxide.
At the same time present methods of etching nitride in plasma or in RIE (reactive ion etch) are somewhat selective toward the oxide but etch silicon much faster than they etch nitride.
The ratio of oxide thickness to nitride thickness therefore cannot be reduced very much because of the risk during nitride etching of also etching all the oxide with consequent etching of and serious damage to the underlying silicon.
Chemical etching of the nitride provides greater selectivity but is preferably in disuse because of high concentration of defects and poor dimension control.
At present, efforts in this area are mainly directed toward finding plasma or R.I.E. nitride etching processes which would be more selective toward the oxide. But satisfactory solutions to the problem are not available.